Mary Douglas argues that all modern cultures have concepts of what is pure and impure, clean and taboo. Purity and danger is an anthropological argument about how these concepts are created.

Douglas' work in the book is wide-ranging and touches on a number of different cultures and examples. Her basic argument is that a major problem that societies face is that many events are seen as ambiguous and anomalous and this material is hard to interpret cognitively and socially. Additionally, reactions to those might be seen as either valid or invalid in the larger community. Purity and taboo, Douglas argues, emerges as a set of shared values that helps us interpret this and that lets up put things clearly either in or out.

By settling for either one interpretation or another, ambiguity can be reduced, the existence of the anomaly can be controlled (or eliminated). Additionally, the rule of avoiding anomalous things strengthens the definition to which the anomalies do not conform, enhances conformity, and can allow the anomalies to be used in ritual.

Essentially, rituals help create clearly defined boundaries around purity and aberrations that both help assure society that the world is more certain and under control and that help provide a set of tools with which to more easily understand the world and act in it.